The present invention relates to a compensation process for a disturbed capacitive circuit, more particularly to a compensation process for a circuit comprising at least one conductor at a specified potential, at least other conductors generating disturbances by capacitive coupling to the said conductor and a disturbance compensation bus capacitively coupled to the said conductor.
The present invention also relates to an application of this process to matrix-drive display screens. It therefore relates to a device for compensating potential for a display screen driven by an electrode array arranged in matrix fashion. It is more particularly relevant to liquid crystal screens, but other screens of the same type may also be used.
The present invention will be described by referring to a matrix-drive display screen, but it can be applied to all disturbed capacitive systems which require compensation, the latter being effected without adding a special measurement line.
In the case of a matrix-drive display screen, the latter consists, in the known manner, of a set of lines and of a set of parallel columns which are activatable, and are arranged perpendicular to one another. This type of screen can operate sequentially, the lines being activated one after the other, while the data are displayed on the columns or vice versa. In the case of line by line sequential operation, the line drive circuit imposes a first selection potential on the chosen line, the other lines being taken to a reference potential. For some of the duration corresponding to the line drive, the column drive circuits impose, on all the columns, a potential dependent on the data to be displayed, and therefore all the column drive circuits change state simultaneously. The simultaneous changes of state therefore produce a capacitive coupling between lines and columns which is all the stronger the larger the difference between drive impedance and load impedance in favour of the latter.
Thus, various solutions have been proposed for compensating for the capacitive couplings between the lines and columns of a matrix screen which uses line or column drive devices, more particularly those exhibiting a high or medium output impedance.
A compensation circuit of this type is described, for example, in French Patent Application No. 94 05987 filed on May 17, 1994 in the name of THOMSON-LCD. In this case, as represented in FIG. 1, an additional electrode is used, namely a column f capacitively coupled by capacitors Cf.sub.j to each of the lines of the screen as well as an additional line g, also capacitively coupled to the columns of the screen, which it crosses in order to effect the compensation. More specifically, FIG. 1 diagrammatically a display screen with matrix array comprising columns i represents (i varying from 1 to n) and lines j (j varying from 1 to m) which are driven by line drive circuits Dj-1 for line j-1, Dj for line j and Dj+1 for line j+1. The column drive circuits i-1, i, i+1 are not represented in the drawing.
In this case, when the output of a line drive circuit Dj of a selection line j is not at low impedance, non-negligible capacitive couplings are brought about by the capacitors labelled Cij (i varying from 1 to n and j varying from 1 to m) between the lines i (i varying from 1 to n) and the columns j (j varying from 1 to m). These capacitive couplings may induce unacceptable voltages on the lines which may even cause a change of state of the transistors located, in the known manner, at the intersections of the lines and columns of the active matrix when the columns are quiescent.
Thus, as represented in FIG. 1, to remedy this drawback and compensate for the couplings, an additional column f is provided, capacitively coupled by the capacitors Cfj to each of the lines j (j varying from 1 to m) of the screen. The value of the capacitor Cfj is chosen to be almost equal to the sum of the coupling capacitances Ci between a line and the columns which they cross. The column f can be activated with each change of polarity on the columns, such as during frame reversals or line reversals for example. Moreover, as represented in FIG. 1, another possible means of driving the column f is to associate therewith, across a comparator 1, a line g capacitively coupled by the capacitors Cgi to the columns i (i varying from 1 to n) which it crosses. This line g makes it possible to detect the couplings with the columns of the screen and to correct the potential of column f by virtue of the capacitive coupling Cfg between column f and line g. In this case, two additional electrodes are necessary, namely the line g for measuring the imbalance due to the capacitive coupling and the column f for compensating for this imbalance.